FIX: amqp_table_clone handle 0-len table correctly

amqp_pool_alloc_bytes() has undefined behavior when a 0-length buffer is
allocated. Properly handles cases where 0-length tables or buffers are
encountered when cloning a table.

This fixes #127
FIX: basic_properties_clone handle 0-len strings

Properly handle 0-length strings in amqp_basic_properties_clone()
FIX: handle 0-len msg body in amqp_read_message